1. Field of the Invention
The present invention relates to a nut runner and clamping method provided with a means for preventing a bolt or the like from loosening immediately after the clamping thereof.
2. Description of the Related Arts
For the execution of clamping a bolt by the use of a nut runner, the bolt is usually screwed with a threaded hole and subjected to a rotation of the motor until a clamping torque T of the bolt reaches a predetermined seating torque TH as shown by a characteristic curve representing the relationship between the clamping torque T and time t in FIG. 2. Under this preliminary clamping, the motor is once deactivated and then reactivated to clamp the bolt until the clamping torque T reaches the predetermined clamping completion torque ST. When the clamping torque T is above the clamping completion torque, the motor comes to a stop to terminate the clamping operation.
At the time of clamping the bolt, a clamping unit of the nut runner, generally interposed between the bolt and the motor, is liable to undergo an accumulation of resilience such as torsion arising from clamping.
In the conventional manner of clamping the bolt, with the resilience being accumulated in the clamping unit, the motor comes to a stop to free the rotation of a rotor of the motor, and hence the rotor is backed in the direction opposite to the direction clamping the bolt by the resilience. As a result, an inverse torque or a counter torque is generated in the motor as shown by a dotted line in FIG. 2. The inverse torque is exerted in the direction loosening the bolt, which results in a looseness of the which has been once clamped by a predetermined torque.
A nut runner provided with a mechanism for canceling the inverse torque is disclosed in Japanese Utility Model Laid-open Publication No. Hei 2-110436. This nut runner is designed to temporarily stop the motor when the clamping torque reaches the clamping completion torque ST, and then reactivate it immediately after the interruption for a forward rotation for a short period of time in such a manner that the inverse torque can be canceled by the forward torque.
According to the conventional measures described above, however, a predetermined constant forward torque is applied only for a short period of time, and hence it is difficult to effectively cancel inverse torques which are different in magnitude. More specifically, if the inverse torque is larger than the forward torque, a torque corresponding to a difference therebetween is exerted in the direction loosening the bolt. On the contrary, if the forward torque is larger than the inverse torque, the excessive forward torque again brings about an accumulation of the resilience such as torsion in the clamping unit of the nut runner, which disadvantageously leads to a regeneration of the inverse torque at the stopping of the motor.